Impurities are implanted into semiconductor devices for a variety of reasons, including introducing electrons and holes into the semiconductor substrate in order to locally change the conductive properties of the substrate. For example, silicon has four electrons in the outer ring. Phosphorus has five electrons in its outer ring, one more than silicon. Boron has three electrons in its outer ring, one fewer electron than silicon. Boron can be used to introduce holes into the substrate. Phosphorous can be used to introduce electrons into the substrate.
To enable implantation, the impurities are implanted as ions having one fewer electron than the neutral species. During the implantation process, the electron deficit can be used to determine how much impurity has been implanted. Specifically, it is not possible to accurately count the number of ions (or atoms) leaving the ion gun. Therefore, a predetermined portion of the ions is directed to an ion counter instead of the semiconductor wafer(s). The ion counter may be embodied has a disk faraday. When an ion strikes the disk faraday, an electron is pulled to the disk faraday in order to neutralize the ion. The number of electrons pulled to the disk faraday is counted using a current meter. It is presumed that the number of ions striking in the disk faraday is proportional to the number of ions striking and entering the semiconductor wafer.
The current (electrons per second) represents the rate at which impurities are introduced into the wafer. If the implanter detects that one area of the wafer is receiving impurities at a slower rate than other areas of the wafer, then the implanter spends more time implanting on the deficient area. In this manner, the implanter can work to achieve uniform total dosing across the surface of the wafer.
When the ions hit the semiconductor wafer, they may destroy a portion of a resist layer formed on the wafer. This process releases an outgas into the implant chamber, which would otherwise kept at a very low pressure. Electrons from the outgas can neutralize a portion of the ions, before the ions reach the disk faraday or the semiconductor wafer. Although the ions are neutralized by the resist outgas (rather than being neutralized at the disk faraday or within the semiconductor wafer), the neutral species is still implanted and still causes the desired change to the substrate. However, because the neutral species contains the correct number of electrons, there is not disk faraday current flow for neutralization. Therefore, the neutral species are not counted.
In order to count the impurities implanted as atoms, rather than ions, a pressure sensor is used. As the pressure increases from resist outgassing, it is presumed that a larger percentage of the impurities are introduced into the wafer as atoms rather than ions.
The following equation represents how pressure is taken into consideration to determine the number of ions implanted.IDISK=IDOSE·e−KP
In the above equation, IDISK is the current flowing to the disk faraday. This current is proportional to the number of ions implanted. IDISK is the rate at which impurities (ions+atoms) are implanted. P is the pressure as sensed by the ion gauge/pressure sensor within the device. K is a factor determined by the engineer and input into the implanter. K represents how a pressure change is presumed to effect ion neutralization.
Instead of, or in addition to, the K-factor shown above, a pressure compensation factor P-COMP can be used. The mathematical relationship between K and P-COMP is as follows:             P      -      COMP        =          100      ⁢              (                              ⅇ                          K              /              10000                                -          1                )              or      K    =          ln      ⁢                          ⁢              (                  1          +                                    P              -              COMP                        100                          )            ⁢                          ⁢              (        10000        )            
Because K and P-COMP are interchangeable through simple math, the term “pressure compensation factor” is used hereinafter to represent both K and P-COMP with the understanding that the two parameters are interchangeable through the above mathematical relationships.
The process chamber is kept at a very low pressure. By detecting pressure increases, the ion gauge is able to calculate the number of ions within the chamber. The chamber is held at a near-vacuum through cryogenic pumps. The conventional ion gauge is located outside of the process chamber, near a cryogenic pump. However, this location reduces the accuracy of the pressure reading for two reasons. First, resist outgassing causes dramatic increase of pressure near the wafer. This high pressure is localized and drops with distance. At the location of the ion gauge, the pressure has dropped significantly, causing an artificially low pressure reading. Second, the ion gauge is in close proximity to the cryogenic pump, which reduces chamber pressure. The cryogenic pump also reduces the pressure reading of the ion gauge.
With high energy implants, the pressure increase is sufficiently high that the implanter can accommodate the pressure inaccuracies. At the outside chamber location, the ion gauge can accurately sense pressure changes produced by implanting high energy impurities, such as arsenic. That is, the high energy of arsenic causes a lot of resist outgassing, and hence a large pressure increases. By positioning the pressure sensor away from the chamber, near the cryogenic pump, pressure changes are reduced to a range where the ion gauge operates efficiently.
While the conventional position is acceptable for high energy impurities, more recent technologies require lower energy implants that work well with the present system. For example, when nitrogen is implanted, the ion gauge detects very little pressure increase. However, nitrogen implantation causes a substantial amount of resist outgassing and beam neutralization, which are not detected by the ion gauge in its current location. One way to address this deficiency is to use a large K value in calculating pressure compensation. The large K value effectively amplifies the pressure readings of the ion gauge. For example, typical P-COMP values for a high energy impurities such as arsenic range from 8 to 60, depending on the beam current, species and energy. For low energy nitrogen beams, a significantly higher P-COMP value is necessary. Perhaps P-COMP would be in excess of 150.
The above high P-COMP values make ion gauge inaccuracies critical in terms of dose accuracy. A high pressure compensation value exposes the process to a non-acceptable risk of dose error. The risk stems from the pressure read out variability and instability that are inherent in any ion gauge. Unstable pressure readings lead to varying dose, even when all other variables are perfectly stable. Experience has shown that for very high P-COMP values, it is difficult to repeatedly produce wafers with the same dosing. High P-COMP values introduce wafer-to-wafer variations.
Another approach to addressing the problem is to reduce the beam current until resist outgassing does not significantly affect the process. Specifically, the beam current is reduced to a point that free electrons produced by resist outgassing are consumed by the cryogenic pump(s) as soon as they are released. In this manner, there are not enough electrons to appreciably neutralize the ion beam. However, reducing the beam current lowers the throughput of the implanter and reduces the tool capacity.